Many aging diseases are based on or associated with extracellular or intracellular deposits of amyloid or amyloid-like proteins that contribute to the pathogenesis as well as to the progression of the disease. The best characterized amyloid protein that forms extracellular aggregates is amyloid beta (Aβ). Diseases involving Abeta (Aβ) aggregates are generally listed as amyloidosis and this includes, but is not limited to, Alzheimer's disease, Lewy body dementia (LBD), hereditary cerebral hemorrhage with amyloidosis (Dutch type), mild cognitive impairment (MCI), progressive supranuclear palsy, multiple sclerosis, inclusion-body myositis (IBM), Creutzfeldt-Jacob disease, Parkinson's disease, HIV-related dementia, amyotropic lateral sclerosis (ALS), inclusion-body myositis (IBM), adult onset diabetes, senile cardiac amyloidosis, endocrine tumors, glaucoma, ocular amyloidosis, primary retinal degeneration, macular degeneration (such as age-related macular degeneration (AMD)), optic nerve drusen, optic neuropathy, optic neuritis, and lattice dystropy.
Other examples of amyloid proteins that form extracellular aggregates are prion, ATTR (transthyretin) or ADan (ADanPP). Diseases associated with the amyloid-like proteins that form extracellular aggregates include, but are not limited to, Creutzfeldt-Jakob disease, associated to prion aggregates, familial senile systemic tenosynovium, associated to ATTR aggregates, and familial dementia (Danish type), associated to ADan aggregates (Sipe et al., Amyloid, 2010, 17, 101-4).
Amyloid-like proteins, that form mainly intracellular aggregates, include, but are not limited to tau, alpha-synuclein, and huntingtin (htt). Diseases involving tau aggregates are generally listed as tauopathies or tauopathies and this includes, but is not limited to, Alzheimer's disease (AD), Creutzfeldt-Jacob disease, dementia pugilistica, Down's Syndrome, Gerstmann-Sträussler-Scheinker disease, inclusion-body myositis, prion protein cerebral amyloid angiopathy, traumatic brain injury, amyotrophic lateral sclerosis, Parkinsonism-dementia complex of Guam, non-Guamanian motor neuron disease with neurofibrillary tangles, argyrophilic grain disease, corticobasal degeneration, diffuse neurofibrillary tangles with calcification, frontotemporal dementia with Parkinsonism linked to chromosome 17, Hallervorden-Spatz disease, multiple system atrophy, Niemann-Pick disease type C, pallido-ponto-nigral degeneration, Pick's disease, progressive subcortical gliosis, progressive supranuclear palsy (PSP), subacute sclerosing panencephalitis, tangle only dementia, postencephalitic Parkinsonism, myotonic dystrophy, tau panencephalopathy, AD-like with astrocytes, certain prion diseases (GSS with tau), mutations in LRRK2, familial British dementia, Hallervorden-Spatz disease, chronic traumatic encephalopathy, familial Danish dementia, frontotemporal lobar degeneration, Guadeloupean Parkinsonism, neurodegeneration with brain iron accumulation, SLC9A6-related mental retardation, and white matter tauopathy with globular glial inclusions etc. (Williams et al., Intern. Med. J., 2006, 36, 652-60).
Amyloid or amyloid-like deposits result from misfolding of proteins followed by aggregation to give β-sheet assemblies in which multiple peptides or proteins are held together by inter-molecular hydrogen-bonds. While amyloid or amyloid-like proteins have different primary amino acid sequences, their deposits often contain many shared molecular constituents, in particular the presence of β-sheet quaternary structures. The reasons for amyloid association with diseases remain largely unclear. A diverse range of protein aggregates, including both those associated and not associated with disease pathologies, have been found to be toxic suggesting that the common molecular features of amyloid are implicated or responsible for disease on-set (Bucciantini et al., Nature, 2002, 416, 507-11). Various multimers of β-sheet aggregated peptides or proteins have also been associated with toxicity for different peptides or proteins ranging from dimers, through to soluble low molecular weight oligomers, protofibrils or insoluble fibrillar deposits.
In AD, some studies suggest that Aβ deposits physically disrupt tissue architecture (Demuro et al., J. Biol. Chem., 2005, 280, 17294-300). However, an emerging consensus implicates prefibrillar intermediates, rather than mature amyloid fibers, in causing cell death (Ferreira et al., IUBMB Life, 2007, 59, 332-45). Other studies have shown that amyloid deposition is associated with mitochondrial dysfunction and a resulting generation of reactive oxygen species (ROS), which can initiate a signaling pathway leading to apoptosis (Kadowaki et al., Cell Death Differ., 2005, 12, 19-24). In addition, local activation of pro-inflammatory pathways thereby leading to the concurrent deposition of activated complement components, acute phase reactants, immune modulators, and other inflammatory mediators seem to play an important role.
Determining the load of amyloid or amyloid-like deposits in living subjects is therefore important in order to help identify subjects at potential risk of developing disease, before disease progression or on-set of symptoms. Knowledge of the amyloid or amyloid-like load in subjects can thus help determine potential measures to prevent or reverse disease progression. Advanced diagnostic techniques are required which are non-invasive and show high sensitivity and selectivity in order to detect and quantify amyloid or amyloid-like load in human tissue or body fluids. For detection of amyloid or amyloid-like deposits, techniques such as those based upon magnetic resonance imaging (MRI) are commonly used. For imaging of amyloid associated with neurological diseases, molecular imaging techniques such as positron emission tomography (PET) (Kadir et al., J. Nucl. Med., 2010, 51, 1418-30, Politis et al., J. Neurol., 2012, 259, 1769-80), single photon emission computed tomography (SPECT) (Ono et al., Int. J. Mol. Imaging, 2011, 543267) or contrast-enhanced magnetic resonance imaging (MRI) (Poduslo et al., Neurobiol. Dis., 2002, 11, 315-29) have been described and enable visualization of amyloid biomarkers.
In order to achieve high target selectivity, molecular probes have been used which recognize and bind to the pathological target. Selectivity for binding to pathological amyloid over other biological entities is therefore a basic requirement of an imaging probe. In order to reduce background signal interference resulting from non-specific off-target binding, and to reduce dosing requirements, imaging compounds should bind with high affinity to their target. Since amyloid or amyloid-like deposits formed from proteins of diverse primary amino acid sequences share a common β-sheet quaternary conformation, molecular probes are required that can differentiate such structures in order to avoid detection of false-positives and mis-diagnosis. In addition, molecular probes must also be designed such that upon administration they can distribute within the body and reach their target. For imaging of amyloid or amyloid-like aggregates associated with neurological disorders such as Alzheimer's disease, Huntington's disease or Parkinson's disease, imaging compounds are required that can penetrate the blood brain barrier and pass into the relevant regions of the brain. For targeting intracellular amyloid-like inclusions, cell permeability is a further requirement of imaging compounds. A further prerequisite in order to avoid unnecessary accumulation of compound which may result in increased risk of unwanted side-effects, is a fast compound wash-out from the brain (or other targeting organ).
Alzheimer's disease (AD) is a neurological disorder primarily thought to be caused by amyloid plaques, an extracellular accumulation of abnormal deposit of Aβ aggregates in the brain. The other major neuropathological hallmarks in AD are the intracellular neurofibrillary tangles (NFT) that originate by the aggregation of the hyperphosphorylated tau protein, phosphorylated tau or pathological tau and its conformers. AD shares this pathology with many neurodegenerative tauopathies, in particular with specified types of frontotemporal dementia (FTD). The tau protein is a freely soluble, “naturally unfolded” protein that binds avidly to microtubuli (MT) to promote their assembly and stability. MT are of major importance for the cytoskeletal integrity of neurons—and thereby for the proper formation and functioning of neuronal circuits, hence for learning and memory. The binding of tau to MT is controlled by dynamic phosphorylation and de-phosphorylation, as demonstrated mainly in vitro and in non-neuronal cells. In AD brain, tau pathology (tauopathy) develops later than, and therefore probably in response to, amyloid pathology, which constitutes the essence of the amyloid cascade hypothesis (Lewis et al., Science, 2001, 293, 1487-1491; Oddo et al., Neuron., 2004, 43, 321-332; Ribe et al., Neurobiol. Dis., 2005, 20(3), 814-22; Muyllaert et al., Rev. Neurol. (Paris), 2006 162, 903-7; Muyllaert et al., Genes Brain and Behav., 2008, Suppl 1, 57-66; Terwel et al., Am. J. Pathol., 2007, 172, 786-798). The exact mechanisms that link amyloid to tau pathology remain largely unknown, but are proposed to involve activation of neuronal signaling pathways that act on or by GSK3 and cdk5 as the major “tau-kinases” (reviewed by Muyllaert et al., Rev. Neurol. (Paris), 2006, 162, 903-7, Muyllaert et al., Genes Brain and Behav. 2008, Suppl 1, 57-66). Even if the tauopathy develops later than amyloid, it is not just an innocent side-effect but a major pathological executer in AD. In experimental mouse models the cognitive defects caused by amyloid pathology are nearly completely alleviated by the absence of protein tau (Roberson et al., Science, 2007, 316(5825), 750-4) and the severity of cognitive dysfunction and dementia correlates with the tauopathy, not with amyloid pathology. Presently, the only definite way to diagnose AD is to identify plaques and tangles in brain tissue in an autopsy after the death of the individual. Therefore, physicians can only make a diagnosis of “possible” or “probable” AD while the person is still alive. Using current methods, physicians can diagnose AD correctly up to 90 percent of the time using several tools to diagnose “probable” AD. Physicians ask questions about the person's general health, past medical problems, and the history of any difficulties the person has carrying out daily activities. Behavioral tests of memory, problem solving, attention, counting, and language provide information on cognitive degeneration and medical tests such as tests of blood, urine, or spinal fluid can provide some further information.
Diagnostic approaches to AD are also challenging due to the knowledge that underlying pathology starts 10 to 20 years before clinical signs of dementia appear (Holtzman et al., Sci. Transl. Med., 2011, 3, 77). In AD, several PET radiopharmaceuticals target Aβ plaques including the FDA-approved tracers [F-18] flutemetamol (Vizamyl®), [F-18] florbetapir (Amyvid®) and 11C-labelled Pittsburgh compound B (PiB). Aβ burden, as measured by PET with these tracers, matches histopathological reports of Aβ distribution in aging and dementia. Indeed, post-mortem analysis of AD patients who had undergone [11C]PiB PET imaging before death suggested a strong correlation between in vivo PiB binding and regional distribution of Aβ plaques (Ikonomovic et al., Brain, 2008, 131, 1630-45). These agents enable the in vivo assessment of brain Aβ pathology and its changes over time, and provide highly accurate, reliable, and reproducible quantitative statements of regional or global Aβ burden in the brain, essential for therapeutic trial recruitment and for the evaluation of anti-Aβ treatments.
Aβ plaques have also been found in the retina of post-mortem eyes from AD patients. Furthermore, non-invasive live optical imaging of Aβ plaques in retina of APP/PS1 transgenic mice was reported following systemic administration of the Aβ-targeting compound curcumin (Koronyo-Hamaoui et al., Neuroimage, 2011, 54 Suppl. 1, S204-S217).
Nevertheless, in humans, Aβ burden does not strongly correlate with cognitive impairment in AD and therefore imaging compounds against other AD biomarkers are needed. The formation of tau aggregates is thought to precede the cognitive symptoms of AD (Lee et al., Trends Mol. Med., 2005, 11, 164-9), hence they represent an additional attractive potential pre-symptomatic marker of AD. Moreover, as NFT load correlates well with the degree of cognitive impairment in AD patients (Arriagada et al., Neurology, 1992, 42, 631-9), imaging agents selective for tau aggregates would provide important information on the tau pathophysiological features in AD and support the development of therapeutic agents targeting tau aggregates. Since Aβ plaques and NFTs colocalize with each other in the neocortex of AD brain, and moreover since the concentrations of Aβ in AD brain are typically 5 to 20 times higher than that of tau (Näslund et al., JAMA, 2000, 283, 1571-7 and Mukactova-Ladinska et al., Am. J. Pathol., 1993, 143, 565-78), imaging probes with high selectivity for NFT are important. This challenge is particularly evident when considering imaging probes such as 2-(1-(6-[2-18F-fluoroethyl)(methyl)amino]-2-naphthyl)ethylidene)malonitrile (18F-FDDNP) which shows equal binding to both AB plaques and NFTs (Shoghi-Jadid et al., Am. J. Geriatr. Psychiatry, 2002, 10, 24-35).
Tau imaging PET probes based on quinoline derivatives, such as [18F]THK-523 (Harada et al., Eur. J. Nucl. Med. Mol. Imaging, 2013, 40, 125-32), hydroxyquinoline derivatives such as [11C]THK-951 (Tago et al., J. Labelled Comp. Radiopharm., 2013, doi: 10.1002/jlcr.3133), and arylquinoline derivatives such as THK-5105 and THK-5117 (Okamura et al., J. Nucl. Med., 2013, 54, 1420) have been described that show high binding affinity for tau pathology in human AD brain sections. Other compounds such as T-807 (Chien et al., J, Alzheimers Dis., 2013, 34, 457-68) and T-808 (Zhang et al., J. Alzheimers Dis., 2012, 31, 601-12, Chien et al., J. Alzheimers Dis., 2014, 38, 171-184) have also been described.
In addition, there is also a need to develop imaging agents suitable for selectively detecting and quantifying the load of amyloid-like deposits associated with other neurodegenerative conditions such as Parkinson's disease (PD) and Huntington's disease (HD).
Parkinson's disease (PD) is the most common neurodegenerative motor disorder. PD is mainly an idiopathic disease, although in at least 5% of the PD patients the pathology is linked to mutations in one or several specific genes (Lesage et al., Hum. Mol. Genet., 2009, 18, R48-59). The pathogenesis of PD remains elusive, however, growing evidence suggests a role for the pathogenic folding of the alpha-synuclein protein that leads to the formation of amyloid-like fibrils. Indeed, the hallmarks of PD are the presence of intracellular alpha-synuclein aggregate structures called Lewy Bodies in the nigral neurons, as well as the death of dopaminergic neurons in the substantia nigra. Alpha-synuclein is a natively unfolded presynaptic protein that can misfold and aggregate into larger oligomeric and fibrillar forms which are linked to the pathogenesis of PD. Recent studies have implicated small soluble oligomeric and protofibrillar forms of alpha-synuclein as the most neurotoxic species (Lashuel et al., J. Mol. Biol., 2002, 322, 1089-102), however the precise role of alpha-synuclein in the neuronal cell toxicity remains to be clarified (review: Cookson, Annu. Rev. Biochem., 2005, 74, 29-52). The PD diagnosis is still solely based on the appearance of symptoms such as tremor, rigidity, and bradykinesia.
Huntington's disease (HD) is a neurodegenerative disorder caused by a mutation in the huntingtin gene (HTT). Typical clinical features of HD include chorea (ancient Greek for “dance”), progressive motor dysfunction, cognitive decline, psychiatric disturbance, and lethality. These debilitating symptoms are thought to be caused by neuronal dysfunction and wide-spread neurodegeneration (Walker et al., Lancet, 2007, 369, 218-28; Ross et al., Medicine (Baltimore), 1997, 76, 305-38). The pathogenesis of HD has been extensively studied and involves the proteolytic cleavage of the HTT protein (Wellington et al., J. Biol. Chem., 1998, 273, 9158-67). HTT cleavage is increased in the mutant HTT and gives rise to N-terminal fragments that acquire abnormal conformations in the cytoplasm or nucleus, and eventually form multimeric assemblies and insoluble high-molecular weight aggregates (Lathrop et al., Proc. Int. Conf. Intell. Syst. Mol. Biol., 1998, 6, 105-14). Post-translational modifications might also affect the mutant HTT conformation, its propensity to aggregate, its cellular localization and clearance, and thus, influence toxicity (Ehrnhoefer et al., Neuroscientist, 2011, 17, 475-492). Misfolding and aggregation initiate a cascade of cytotoxic events. Formal diagnosis of HD onset in an individual at risk, i.e. carrying the mutated HTT gene (Huntington Study Group, Mov. Disord., 1996, 11, 136-42) is made on the motor signs like bradykinesia, chorea, dystonia, or incoordination (Marder et al., Neurology, 2000, 54, 452-58).
WO 2011/128455 refers to specific compounds which are suitable for treating disorders associated with amyloid proteins or amyloid-like proteins. US 2012/0302755 relates to certain imaging agents for detecting neurological dysfunction. Further compounds for the diagnosis of neurodegenerative disorders on the olfactory epithelium are discussed in WO 2012/037928.